AP_Avoidance: Add missing const in member functions

Signed-off-by: Patrick José Pereira <patrickelectric@gmail.com>

libraries/AP_Avoidance/AP_Avoidance.cpp

@@ -609,7 +609,7 @@ void AP_Avoidance::handle_msg(const mavlink_message_t &msg)
 }
 
 // get unit vector away from the nearest obstacle
-bool AP_Avoidance::get_vector_perpendicular(const AP_Avoidance::Obstacle *obstacle, Vector3f &vec_neu)
+bool AP_Avoidance::get_vector_perpendicular(const AP_Avoidance::Obstacle *obstacle, Vector3f &vec_neu) const
 {
     if (obstacle == nullptr) {
         // why where we called?!

libraries/AP_Avoidance/AP_Avoidance.h

@@ -140,7 +140,7 @@ protected:
     bool get_destination_perpendicular(const AP_Avoidance::Obstacle *obstacle, Vector3f &newdest_neu, const float wp_speed_xy, const float wp_speed_z, const uint8_t _minimum_avoid_height);
 
     // get unit vector away from the nearest obstacle
-    bool get_vector_perpendicular(const AP_Avoidance::Obstacle *obstacle, Vector3f &vec_neu);
+    bool get_vector_perpendicular(const AP_Avoidance::Obstacle *obstacle, Vector3f &vec_neu) const;
 
     // helper functions to calculate destination to get us away from obstacle
     // Note: v1 is NED